Group III nitride crystals such as aluminum nitride, gallium nitride and indium nitride have a wide range of band gap energy, and the band gap energy values of these are about 6.2 eV, about 3.4 eV and about 0.7 eV, respectively. These group III nitride crystals can provide a mixed crystal semiconductor having desired composition, and it is possible to control the band gap energy of the mixed crystal based on the above values because the band gap energy of the mixed crystal corresponds to its composition.
Therefore, it is theoretically possible to fabricate a light emitting device which emits a wide range of light from infrared light to ultraviolet light by using a group III nitride crystal. The development of a light emitting device comprising an aluminum-based group III nitride crystal, mainly an aluminum gallium nitride mixed crystal is now under way energetically. The emission of light having short-wavelength in an ultraviolet range is made possible by using an aluminum-based group III nitride crystal, whereby light emitting sources such as an ultraviolet light emitting diode for white light sources, an ultraviolet light emitting diode for sterilization, a laser which can be used to read and write a high-density optical disk memory and a communication laser can be manufactured.
A light emitting device comprising an aluminum-based group III nitride crystal (to be also referred to as “aluminum-based group III nitride light emitting device” hereinafter) is manufactured by forming a laminated structure consisting of semiconductor single crystal thin films, each having a thickness of several microns, specifically a p-type semiconductor layer, a light emitting layer and an n-type semiconductor layer on a substrate sequentially like a conventional light emitting device. This laminated structure is generally formed by a crystal growth method such as molecular beam epitaxy (MBE) or metalorganic vapor phase epitaxy (MOVPE). Researches are now being made to form a preferred laminated structure as a light emitting device by the above method for the manufacture of an aluminum-based group III nitride light emitting device (refer to Japanese Journal of Applied Physics, Vol. 44, No. 10, 2005, pp. 7191-7206).
As the means of forming an aluminum-based group III nitride crystal layer, there is known hydride vapor phase epitaxy (HVPE) (refer to JP-A 2003-303774) besides the above MBE and MOVPE. Although HVPE is more advantageous than MBE and MOVPE in production cost and film forming rate, it has the difficulty of controlling the thickness of each film accurately. Therefore, the above method is rarely employed as the means of forming the crystal layers of a semiconductor light emitting device.
As a substrate which is used in an ultraviolet light emitting device, a sapphire substrate is now generally used from the viewpoints of crystal quality as a substrate, ultraviolet light transmission, mass productivity and cost. However, when a sapphire substrate is used, a problem occurs due to differences in physical properties between the sapphire substrate and aluminum gallium nitride forming a semiconductor laminated film. For instance, due to a difference in lattice constant between the substrate and the semiconductor laminated film (misfit), a crystal defect called “dislocation” is introduced into the semiconductor laminated film. It is generally known that the light emitting performance of a semiconductor laminated film deteriorates or the service life of a device is shortened when a dislocation exists. To prevent the formation of the dislocation by such a misfit, it is proposed to use a template substrate having an aluminum-based group III nitride crystal film on a single crystal substrate such as a sapphire substrate or a self-supporting substrate composed of only an aluminum-based group III nitride crystal.
To manufacture the template substrate or the self-supporting substrate, an aluminum-based group III nitride crystal must be grown by a vapor-phase growth on a single crystal substrate such as a sapphire substrate. MOVPE has been generally employed as a vapor-phase growth for this purpose because a high-quality crystal is obtained. An aluminum-based group III nitride crystal self-supporting substrate can also be obtained by using improved HVPE (refer to JP-A 2005-252248).
In the conventional HVPE, an apparatus in which a substrate installed in a quartz glass reactor tube is heated from the outside of the reaction tube by resistance heating as disclosed by JP-A 2003-303774 is generally used. When this type of apparatus is used, the upper limit of heating temperature is limited to the heat resistance temperature of the quartz glass reactor tube and even when a quartz glass reactor tube having the highest durability is used, it is difficult to carry out crystal growth at 1,200° C. for a long time. However, as disclosed by JP-A 2005-252248, a crystal growth can be conducted at a high substrate temperature of 950 to 1,700° C. by improving the apparatus, and a high-quality aluminum-based group III nitride crystal layer can also be obtained by using a sapphire substrate having an aluminum-based group III nitride crystal film obtained by MOPVE thereon.
As means of obtaining an aluminum-based group III nitride crystal having high crystallinity by HVPE, there is known a method in which a group III halide gas and a nitrogen source gas are reacted with each other while a substrate such as a sapphire is maintained at a low temperature of 300 to 550° C. and then the substrate is heated at a temperature of 1,100 to 1,600° C. to react these gases with each other (refer to JP-A 2006-335607).
To obtain a high-quality aluminum-based group III nitride crystal layer having a smooth surface by the method disclosed by JP-A 2005-252248, a single crystal substrate such as a sapphire substrate having an aluminum-based group III nitride crystal film formed by MOVPE thereon must be used as a substrate. However, as MOVPE has such demerits that the raw materials are expensive and that the film forming rate is low, the use of this method is not always satisfactory from the viewpoints of production cost and efficiency. In the above method, after a film is formed by MOVPE, HVPE must be employed by changing the apparatus or the raw materials. Therefore, the operation becomes complicated, and impurities which contaminate the substrate may be included in the operation process.
Additionally, by the investigation of the inventors of the present invention, it is revealed that the aluminum-based group III nitride crystal obtained by the method disclosed in JP-A 2006-335607 is excellent in crystallinity but its surface smoothness is not satisfactory.